Many recent apparatuses adopt wireless power transmission systems that wirelessly transmit electric power in a noncontact manner by using mutual inductance between a power transmitting coil and a power receiving coil. A power transmitting coil used in such a wireless power transmission system includes a ferrite core, a coil wire wound around the ferrite core, and a resin covering the ferrite core and the coil wire. The coil wire is a stranded wire having low loss, such as a Litz wire.
When the ferrite core with the Litz wire wound therearound is covered with the resin, a space between turns of the Litz wire or a vicinity of the Litz wire may not be filled with the resin, and a void (cavity) may be formed. If a void is formed in the resin, the electrical field can be concentrated in the void to produce a discharge, thereby causing a dielectric breakdown. In addition, there is a possibility that heat is not uniformly diffused, the thermal conductivity decreases, and the resin deteriorates.